Predator crown-of-thorns starfish (Acanthaster planci) outbreak, mass mortality of corals, and cascading effects on reef fish and benthic communities.

Mohsen Kayal,Céline Stievenart,Julie Vercelloni,Yannick Chancerelle,Serge Planes,Pauline Bosserelle,Sylvie Geoffroy,Mehdi Adjeroud,François Michonneau,Thierry Lison De Loma,Lucie Penin

doi:10.1371/journal.pone.0047363

Abstract

Outbreaks of the coral-killing seastar Acanthaster planci are intense disturbances that can decimate coral reefs. These events consist of the emergence of large swarms of the predatory seastar that feed on reef-building corals, often leading to widespread devastation of coral populations. While cyclic occurrences of such outbreaks are reported from many tropical reefs throughout the Indo-Pacific, their causes are hotly debated, and the spatio-temporal dynamics of the outbreaks and impacts to reef communities remain unclear. Based on observations of a recent event around the island of Moorea, French Polynesia, we show that Acanthaster outbreaks are methodic, slow-paced, and diffusive biological disturbances. Acanthaster outbreaks on insular reef systems like Moorea's appear to originate from restricted areas confined to the ocean-exposed base of reefs. Elevated Acanthaster densities then progressively spread to adjacent and shallower locations by migrations of seastars in aggregative waves that eventually affect the entire reef system. The directional migration across reefs appears to be a search for prey as reef portions affected by dense seastar aggregations are rapidly depleted of living corals and subsequently left behind. Coral decline on impacted reefs occurs by the sequential consumption of species in the order of Acanthaster feeding preferences. Acanthaster outbreaks thus result in predictable alteration of the coral community structure. The outbreak we report here is among the most intense and devastating ever reported. Using a hierarchical, multi-scale approach, we also show how sessile benthic communities and resident coral-feeding fish assemblages were subsequently affected by the decline of corals. By elucidating the processes involved in an Acanthaster outbreak, our study contributes to comprehending this widespread disturbance and should thus benefit targeted management actions for coral reef ecosystems.

Highlights

The crown-of-thorns seastar Acanthaster planci (Figure 1) is the major natural enemy of reef-building corals [1,2]
Cascading effects of Acanthaster outbreaks usually spread to the entire reef ecosystem and commonly lead to increases in benthic algae, a loss of coral-feeding assemblages, an overall collapse of reef structural complexity, and a decline in biodiversity and productivity [6,9,10,11]
We describe an Acanthaster outbreak that occurred during the last decade in French Polynesia (South Pacific), a region where these disturbances occur with a periodicity of,20 years and, along with bleaching events and cyclones, are the major drivers of community dynamics on coral reefs [27]

Summary

Introduction

The crown-of-thorns seastar Acanthaster planci (Figure 1) is the major natural enemy of reef-building corals [1,2]. This specialized coral-feeder is found on tropical reefs across the planet, except in the Atlantic Ocean. Populations of Acanthaster commonly display cyclic oscillations between extended periods of low-density with individuals scarcely distributed among large reef areas, and brief episodes of unsustainably high densities commonly termed ‘outbreaks’ [3]. These outbreaks are among the most destructive disturbances observed on tropical reefs [4,5].

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Conclusion